Electronic circuit assemblies are often required to be capable of surviving in hostile operating environments, including those commonly found in automotive and aerospace applications. Such assemblies often employ surface-mount (SM) integrated circuit (IC) devices, which are generally characterized as being electrically and mechanically attached to the substrate of an electronic circuit assembly with a number of terminals or leads that are soldered to conductors on the surface of the substrate, which may be a ceramic substrate, laminate board, flex circuit, or a silicon substrate. A prominent example of a SM IC is a flip chip, which has bead-like terminals typically in the form of solder bumps on a surface of the chip. The solder bumps of a flip chip are generally formed by selectively depositing a solder composition, such as a solder paste containing a solder alloy and binder, on the flip chip, and then reflowing the solder material by heating the material above its liquidus temperature so that the molten material coalesces to form the solder bumps on the surface of the chip. After solidifying, the solder bumps can be precisely registered with their corresponding conductors on a substrate, and then reheated above the solder material's liquidus temperature in order to both bond the chip to the substrate and electrically interconnect the flip chip circuit to the conductor pattern. The temperature at which the solder is reflowed to form the solder bump and later reflowed to mount the flip chip is referred to as the reflow temperature.
There is a desire in the electronics industry to limit the use of lead-containing materials due to environmental concerns. In addition, there are serious reliability concerns for some flip chip devices due to the alpha particles emitted by lead-containing bump alloys. Lead-containing alloys usually contain a certain amount of lead isotopes, such as Pb-210 or Pb-214. These isotopes are very difficult to remove during typical lead metal refining processes unless extremely expensive laser plasma isotope separation processes are used. These isotopes are chemically unstable, and will emit alpha particles during the normal radioactive decay process. Alpha particles released through radioactive decay of Pb-210 and Pb-214 can carry an energy of up to 5.4 MeV and 7.8 MeV, respectively. A S MeV alpha particle could penetrate up to 25 micrometers of silicon and generate 1.4 million electron-hole pairs. If the electron-hole accumulation exceeds the critical charge for a circuit such as a cell in a DRAM, a soft error could occur in the memory section of the device. High purity Pb-free alloys usually do not contain heavy elements and are therefore free of radioactive isotopes.
There are many commercially available Pb-free alloys, including Sn-52In, Bi-42Sn, Sn-20In-2.8Ag, Sn-3Ag-2Bi, Sn-5Ag, Sn-8.5Sb, Sn-1Cu, Sn-3.5Ag, Sn-2.5Ag-0.8Cu-0.5Sb, Sn-4.8Bi-3.4Ag, Sn-9Zn and Sn-8.8In-7.6Zn. However, none of these alloys meet the requirements for automotive applications with flip chips on laminate boards. Many of the alloys contain bismuth. Though bismuth is environmentally friendly, it also contains radioactive isotopes that are difficult to remove. Consequently, bismuth-containing alloys are not typically suitable for flip chip packaging applications due to the potential for alpha-particle induced reliability concerns.
A typical requirement for automotive applications is to withstand 150.degree. C. junction temperatures for an extended period of time (e.g., 2000 hours continuous operation at 150.degree. C.). This requirement excludes all Pb-free alloys with solidus temperatures under about 170.degree. C., such as Sn-52In, Bi-42Sn and Sn-20In- 2.8Ag. Though the reported solidus temperature of the Sn-20In-2.8Ag alloy is about 175.degree. C., this alloy has been unable to pass a -50.degree. C./+150.degree. C. thermal cycle test due to the presence of a eutectic In/Sn phase having a melting temperature of about 120.degree. C. Accordingly, 120.degree. C. is the effective solidus temperature of this alloy.
For cost saving purposes, there is a trend to mount flip chips directly on laminate boards along with other SM components using a typical eutectic component reflow process with peak reflow temperatures of about 225.degree. C. to about 240.degree. C. In this case, flip chips are treated as another standard SM component, and are attached to the substrate with one eutectic Sn/Pb component reflow. This requires an alloy with a liquidus temperature of about 200.degree. C. or lower, and excludes the remaining Pb-free alloys noted above except for Sn-9Zn and Sn-8.8In-7.6Zn. Solder pastes formed with these last two alloys are very difficult to process due to a well-known susceptibility to zinc oxidation. This oxidation problem can be problematic and cause poor bumping and substrate assembly yields.
Accordingly, it would be desirable if a lead-free solder composition were available that was capable of forming solder joints that can reliably withstand applications with maximum IC junction temperatures of 150.degree. C., yet can be assembled to laminate boards with other SM components at a peak reflow temperature of less than 240.degree. C.